The evolution of wind power generation technology is currently aimed at designing aerogenerators with increasingly larger power outputs, which implies an increase in the size of the constituent parts of the aerogenerators. The development of higher towers must be carried out taking into account a set of design criteria, such as load, resistance of the material and dynamic performance, in addition to construction, transport and installation conditions.
Towers up to 80 m in height are usually manufactured from tubular metal sections. However, it has been verified that more resistant and rigid structures are required as of this height, due to the increased load they must bear and the resonant frequency of the structure. A possible solution is the use of lower metallic sections of greater diameter and/or thickness. However, this has different drawbacks related to transport problems and an excessive increase in manufacturing costs. For this reason, the use of concrete to build the tower is an attractive alternative. However, there is a wide range of problems related to the joining of the last concrete section to the gondola of the aerogenerator and particularly to the bearing that allows the gondola to rotate around a vertical rotation axis, in order for the turbine of the aerogenerator to orient itself in accordance with the direction of the wind. This bearing is normally called “yaw bearing.”
The yaw bearing has a diameter of between 2 m and 2.3 m, depending on the size of the machine, and supports both the weight of the gondola and aerogenerator rotor and the loads deriving from the action of the wind on the rotor. For this reason, the yaw bearing imposes stringent requirements on the surface that supports it, basically in terms of flatness and resistance.
In relation to flatness, the tolerance relative to surface flatness is typically 0.25 mm, which is impossible to obtain in concrete parts. Additionally, with the object of facilitating transport of the tower sections that constitute a concrete tower, each section may in turn be formed by several adjacent modules fixed together by vertical joints, such as for example in EP1474579. In these cases, the flatness requirement for yaw bearing seating would require a perfect vertical alignment of the adjacent modules, which is difficult to achieve given the dimensions (greater than 15 m) and weight (tens of tonnes) of said modules.
With regard to resistance, and particularly when the final section of the tower that supports the bearing is made of concrete, there is the added problem of distributing tension as evenly as possible. We must point out that the bearing support surface is considerably smaller than the support surface of the concrete section and that, in addition to axial tension, tangential tension also appears, caused by the action of the wind and rotation of the rotor. Additionally, the yaw bearing is manufactured from steel, a substantially more resistant and rigid material than the conventional concrete used in the construction of aerogenerator towers.
For these reasons, use of an intermediate part that is sufficiently flat and distributes stress evenly in the concrete is required. At present, a cylindrical metal part having upper and lower clamps that serve to join it respectively to the yaw bearing and to the concrete section by means of screws or bolts is normally used. For example, EP1474579 discloses a part of this kind, frequently called “mast” in the state of the art.
In order to improve the joint between the cylindrical part and the concrete section, joining systems have also been disclosed wherein the lower portion of the metal cylinder is embedded in the concrete. However, a drawback of this system is that the stress transmitted by the cylindrical part to the concrete may lead to the appearance of cracks in stress concentration points due to fatigue. EP1654460 and EP1947328 describe cylindrical joining parts of this kind.